1. Field of the Invention
The present invention relates generally to the field of power amplifiers; and, more particularly, to a method and apparatus for level shifting the control signal that sets the Gate voltage in a power amplifier.
2. Description of the Prior Art
Cellular telephones used in mobile telecommunications systems; for example, mobile telecommunications systems which operate in accordance with GSM (Global System for Mobile Communications) specifications; contain a power amplifier for amplifying a transmitted signal. The power amplifier may include one or more stages which may be implemented by one or more MESFET (Metal Semiconductor Field Effect Transistor) transistors. In many power amplifiers, for example, in power amplifiers which use MESFETs, it is necessary to level shift the control signal which sets the Gate voltage of the power amplifier. The reason for this is that MESFETs have a negative pinch-off voltage (xe2x88x922V is one example).
FIG. 1 illustrates a known circuit for level shifting the control signal of a power amplifier such as a power amplifier using MESFETs. In particular, FIG. 1 illustrates, a level shifting circuit (sometimes referred to herein as a xe2x80x9clevel shifterxe2x80x9d), generally designated by reference number 10, which may be incorporated in the power amplifier system of a cellular telephone, schematically illustrated by dashed box 12. As shown in FIG. 1, the level shifting circuit 10 includes a control voltage (Vcontrol) 18, followed by a resister 20 having a resistance of 250 ohms, and a series of diodes 22 as are needed to achieve a desired voltage shift. In FIG. 1, circuit 10 includes four diodes 22a-22d; and each of the four diodes lowers the control voltage by approximately 0.6V.
In order to provide the desired voltage drop across each of the diodes 22a-22d, a current source 14 connects the last diode 22d of the series of diodes to a low voltage supply (Vneg) 24 of, for example, xe2x88x923.6V. Because of the presence of the current source, the level shifter 10 is insensitive to changing Vneg. The voltage to the Gate is taken across the current source 14. In the exemplary circuit illustrated in FIG. 1, the current source 14 comprises a transistor 15 and is made by connecting the gate and source terminals 26 and 28, respectively, of the transistor to Vneg and the drain 30 of the transistor to the last diode 22d in the series of diodes 22.
In a cellular telephone that includes a power amplifier incorporating a level shifter such as level shifting circuit 10, a problem is encountered in that as the voltage in the telephone gets lower, the dynamics of the Vcontrol (maximum/minimum voltage) also gets lower. This is undesirable as it is important that control of the gate voltage remain the same as when there was a higher voltage in the telephone if the MESFET transistors are not changed.
Another problem with respect to using the level shifter illustrated in FIG. 1 is encountered in power amplifiers having more than one stage. In such power amplifiers, when ramping up the Gate voltages at the same slope, there is a risk that the output stage of the power amplifier could show a non-monotonic behavior when the RF-signal begins to saturate the MESFET transistor implementing the output stage. This is undesirable as it is important that the slope be monotonic in order for the power amplifier to be controllable with a power control circuit. This non-monotonic behavior comprises a dip in the Gate voltage, and is primarily caused by a high series resistance in the level shifter output impedance, and also because the input power increases too fast.
The present invention provides a level shifting circuit for level shifting a control signal that sets a Gate voltage of a power amplifier. A level shifting circuit according to the present invention includes circuitry for adding gain to the level shift.
By adding gain to the level shift according to the present invention, it becomes possible to obtain higher dynamics on the voltage control to thereby provide improved control of the Gate voltage of the power amplifier.
According to a presently preferred embodiment of the invention, the level shifting circuit includes a control voltage followed by a resister and at least one diode to achieve a desired voltage shift. A current source connects the last diode of the at least one diode to a low voltage supply. The circuitry for adding gain comprises a common gate amplifier connected to a portion of the control voltage and to the resister connected between the control voltage and the one or more diodes such that the current from the common gate amplifier is fed back to the resister. As a result, the change in the Gate voltage of the power amplifier can be made greater than the change in the control voltage.
The level shifting circuit according to the present invention can be advantageously used in a power amplifier that includes more than one stage. As indicated previously, in a multi-stage power amplifier, the output stage could show non-monotonic behavior, i.e., a dip in the gate voltage, when the RF-signal begins to saturate the MESFET transistor of the output stage; and this can interfere with the power amplifier being controllable by a power control circuit. By adding gain to the level shift, it is possible to still get to the optimum biasing point at full output power which is often at a higher voltage than where the dip occurs with a prior art circuit such as illustrated in FIG. 1.
Yet further advantages and specific features of the present invention will become apparent hereinafter in conjunction with the following detailed description of presently preferred embodiments of the invention.